MAPKKK dual leucine zipper-bearing kinases (DLKs) are regulators of synaptic development and axon regeneration. The mechanisms underlying their activation are not fully understood. Here, we show that ...C. elegans DLK-1 is activated by a Ca2+-dependent switch from inactive heteromeric to active homomeric protein complexes. We identify a DLK-1 isoform, DLK-1S, that shares identical kinase and leucine zipper domains with the previously described long isoform DLK-1L but acts to inhibit DLK-1 function by binding to DLK-1L. The switch between homo- or heteromeric DLK-1 complexes is influenced by Ca2+ concentration. A conserved hexapeptide in the DLK-1L C terminus is essential for DLK-1 activity and is required for Ca2+ regulation. The mammalian DLK-1 homolog MAP3K13 contains an identical C-terminal hexapeptide and can functionally complement dlk-1 mutants, suggesting that the DLK activation mechanism is conserved. The DLK activation mechanism is ideally suited for rapid and spatially controlled signal transduction in response to axonal injury and synaptic activity.
► DLK-1 short isoform acts as an endogenous inhibitor to DLK-1 functions ► A novel conserved hexapeptide is required for DLK-1 activation ► Ca2+ regulates DLK-1 isoform-specific interactions ► Human MAP3K13 contains the same hexapeptide and complements DLK-1 function in vivo
MAPKKK dual leucine zipper-bearing kinases (DLKs) are regulators of synaptic development and axon regeneration. Yan and Jin show that C. elegans DLK-1 is activated by a Ca2+-dependent switch from inactive heteromeric to active homomeric protein complexes.
Stress-associated p38 and JNK mitogen-activated protein (MAP) kinase signaling cascades trigger specific cellular responses and are involved in multiple disease states. At the root of MAP kinase ...signaling complexity is the differential use of common components on a context-specific basis. The roundworm Caenorhabditis elegans was developed as a system to study genes required for development and nervous system function. The powerful genetics of C. elegans in combination with molecular and cellular dissections has led to a greater understanding of how p38 and JNK signaling affects many biological processes under normal and stress conditions. This review focuses on the studies revealing context specificity of different stress-activated MAPK components in C. elegans.
The correct wiring of neuronal circuits depends on outgrowth and guidance of neuronal processes during development. In the past two decades, great progress has been made in understanding the ...molecular basis of axon outgrowth and guidance. Genetic analysis in Caenorhabditis elegans has played a key role in elucidating conserved pathways regulating axon guidance, including Netrin signaling, the slit Slit/Robo pathway, Wnt signaling, and others. Axon guidance factors were first identified by screens for mutations affecting animal behavior, and by direct visual screens for axon guidance defects. Genetic analysis of these pathways has revealed the complex and combinatorial nature of guidance cues, and has delineated how cues guide growth cones via receptor activity and cytoskeletal rearrangement. Several axon guidance pathways also affect directed migrations of non-neuronal cells in C. elegans, with implications for normal and pathological cell migrations in situations such as tumor metastasis. The small number of neurons and highly stereotyped axonal architecture of the C. elegans nervous system allow analysis of axon guidance at the level of single identified axons, and permit in vivo tests of prevailing models of axon guidance. C. elegans axons also have a robust capacity to undergo regenerative regrowth after precise laser injury (axotomy). Although such axon regrowth shares some similarities with developmental axon outgrowth, screens for regrowth mutants have revealed regeneration-specific pathways and factors that were not identified in developmental screens. Several areas remain poorly understood, including how major axon tracts are formed in the embryo, and the function of axon regeneration in the natural environment.
A determinant of axon regeneration is the intrinsic growth ability of injured neurons, which dictates a battery of injury responses in axons and cell bodies. While some of these regulatory mechanisms ...are evolutionarily conserved, others are unique to the mammalian central nervous system (CNS) where spontaneous regeneration usually does not occur. Here we examine our current understanding of these mechanisms at cellular and molecular terms and discuss their potential implications for promoting axon regeneration and functional recovery after nerve injury.
Dependent on animal species, age, and neuronal type, injured axons exhibit distinct regenerative responses. In this Review, He and Jin discuss the current understanding of mechanisms regulating axon regeneration and their implications for the development of neural repair strategies.
Axons of adult Caenorhabditis elegans neurons undergo robust regenerative growth after laser axotomy. Here we show that axotomy of PLM sensory neurons triggers axonal calcium waves whose amplitude ...correlates with the extent of regeneration. Genetic elevation of Ca(2+) or cAMP accelerates formation of a growth cone from the injured axon. Elevated Ca(2+) or cAMP also facilitates apparent fusion of axonal fragments and promotes branching to postsynaptic targets. Conversely, inhibition of voltage-gated calcium channels or calcium release from internal stores reduces regenerative growth. We identify the fusogen EFF-1 as critical for axon fragment fusion and the basic leucine zipper domain (bZip) protein CREB (cAMP response element-binding protein) as a key effector for branching. The effects of elevated Ca(2+) or cAMP on regrowth require the MAPKKK (mitogen-activated protein kinase kinase kinase) DLK-1. Increased cAMP signaling can partly bypass the requirement for the bZip protein CEBP-1, a downstream factor of the DLK-1 kinase cascade. These findings reveal the relationship between Ca(2+)/cAMP signaling and the DLK-1 MAPK (mitogen-activated protein kinase) cascade in regeneration.
Coordinating the balance between development and stress responses is critical for organismal survival. However, the cellular signaling controlling this mechanism is not well understood. In ...Caenorhabditis elegans, it has been hypothesized that a genetic network regulated by NIPI-3/Tibbles may control the balance between animal development and immune response. Using a nipi-3(0) lethality suppressor screen in C. elegans, we reveal a novel role for N-terminal acetyltransferase C complex natc-1/2/3 and histone deacetylase hda-4, in the control of animal development. These signaling proteins act, at least in part, through a PMK-1 p38 MAP kinase pathway (TIR-1-NSY-1-SEK-1-PMK-1), which plays a critical role in the innate immunity against infection. Additionally, using a transcriptional reporter of SEK-1, a signaling molecule within this p38 MAP kinase system that acts directly downstream of C/EBP bZip transcription factor CEBP-1, we find unexpected positive control of sek-1 transcription by SEK-1 along with several other p38 MAP kinase pathway components. Together, these data demonstrate a role for NIPI-3 regulators in animal development, operating, at least in part through a PMK-1 p38 MAPK pathway. Because the C. elegans p38 MAP kinase pathway is well known for its role in cellular stress responses, the novel biological components and mechanisms pertaining to development identified here may also contribute to the balance between stress response and development.
Growth cone guidance and synaptic plasticity involve dynamic local changes in proteins at axons and dendrites. The
Dual-
Leucine zipper Kinase MAPKK
K (DLK) has been previously implicated in ...synaptogenesis and axon outgrowth in
C. elegans and other animals. Here we show that in
C. elegans DLK-1 regulates not only proper synapse formation and axon morphology but also axon regeneration by influencing mRNA stability. DLK-1 kinase signals via a MAPKAP kinase, MAK-2, to stabilize the mRNA encoding CEBP-1, a bZip protein related to CCAAT/enhancer-binding proteins, via its 3′UTR. Inappropriate upregulation of
cebp-1 in adult neurons disrupts synapses and axon morphology. CEBP-1 and the DLK-1 pathway are essential for axon regeneration after laser axotomy in adult neurons, and axotomy induces translation of CEBP-1 in axons. Our findings identify the DLK-1 pathway as a regulator of mRNA stability in synapse formation and maintenance and also in adult axon regeneration.
The PIWI-interacting RNA (piRNA) pathway has long been thought to function solely in the germline, but evidence for its functions in somatic cells is emerging. Here we report an unexpected role for ...the piRNA pathway in Caenorhabditis elegans sensory axon regeneration after injury. Loss of function in a subset of components of the piRNA pathway results in enhanced axon regrowth. Two essential piRNA factors, PRDE-1 and PRG-1/PIWI, inhibit axon regeneration in a gonad-independent and cell-autonomous manner. By smFISH analysis we find that prde-1 transcripts are present in neurons, as well as germ cells. The piRNA pathway inhibits axon regrowth independent of nuclear transcriptional silencing but dependent on the slicer domain of PRG-1/PIWI, suggesting that post-transcriptional gene silencing is involved. Our results reveal the neuronal piRNA pathway as a novel intrinsic repressor of axon regeneration.
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•piRNA pathway inhibits axon regeneration in C. elegans•piRNA factors act cell-autonomously in neurons to limit axon regeneration•Transcripts of the piRNA factor prde-1 are present in neurons•The slicer activity of PRG-1/PIWI is required to inhibit axon regrowth
Kim et al. discovered that specific factors in the PIWI-interacting small RNA (piRNA) pathway inhibit axon regeneration in C. elegans. Transcripts of the piRNA factor prde-1 are present in neurons, and piRNA factors function cell-autonomously to limit axon regeneration.
The microtubule (MT) cytoskeleton of a mature axon is maintained in a stabilized steady state, yet after axonal injury it can be transformed into a dynamic structure capable of supporting axon ...regrowth. Using Caenorhabditis elegans mechanosensory axons and in vivo imaging, we find that, in mature axons, the growth of MTs is restricted in the steady state by the depolymerizing kinesin-13 family member KLP-7. After axon injury, we observe a two-phase process of MT growth upregulation. First, the number of growing MTs increases at the injury site, concomitant with local downregulation of KLP-7. A second phase of persistent MT growth requires the cytosolic carboxypeptidase CCPP-6, which promotes Δ2 modification of α-tubulin. Both phases of MT growth are coordinated by the DLK-1 MAP kinase cascade. Our results define how the stable MT cytoskeleton of a mature neuron is converted into the dynamically growing MT cytoskeleton of a regrowing axon.
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► Axon injury triggers a two-phase process of microtubule (MT) growth upregulation ► The depolymerizing kinesin-13 KLP-7 is locally downregulated at the axon stump ► Tubulin posttranslational modifications regulate axon regeneration ► The DLK-1 pathway regulates both axonal kinesin-13 and tubulin modifications
Axon regeneration after injury involves conversion of the stable microtubule (MT) cytoskeleton of a mature axon into the dynamic cytoskeleton of a regrowing process. Using C. elegans in vivo imaging, Ghosh-Roy et al. show that the DLK-1 pathway regulates this transition via an MT depolymerizing kinesin and tubulin posttranslational modifications.
Cell- or network-driven oscillators underlie motor rhythmicity. The identity of
oscillators remains unknown. Through cell ablation, electrophysiology, and calcium imaging, we show: (1) forward and ...backward locomotion is driven by different oscillators; (2) the cholinergic and excitatory A-class motor neurons exhibit intrinsic and oscillatory activity that is sufficient to drive backward locomotion in the absence of premotor interneurons; (3) the UNC-2 P/Q/N high-voltage-activated calcium current underlies A motor neuron's oscillation; (4) descending premotor interneurons AVA, via an evolutionarily conserved, mixed gap junction and chemical synapse configuration, exert state-dependent inhibition and potentiation of A motor neuron's intrinsic activity to regulate backward locomotion. Thus, motor neurons themselves derive rhythms, which are dually regulated by the descending interneurons to control the reversal motor state. These and previous findings exemplify compression: essential circuit properties are conserved but executed by fewer numbers and layers of neurons in a small locomotor network.
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